Vim: fix `d` quickfix mapping in VISUAL mode
Vim: fix `d` quickfix mapping in VISUAL mode

Oh, Vim... The reason why my function was getting called once per line
is explained in `:h a:firstline` (why didn't I think to look there?):

> If [range] is excluded, ":{range}call" will call the
> function for each line in the range, with the
> cursor on the start of each line.

With this commit, VISUAL is handled nicely, as is NORMAL mode. That
leaves only motion support to make this truly seamless.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent